Modern tool icons

[matterhorn103]

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[matterhorn103]

My proposal for a visual refresh of the toolbar icons, to make them more modern and improve clarity of function and distinguishability as discussed on the forum.

Might well be further changes based on further feedback there but for now I thought I'd open a PR so others can try them out.

The icons are now all SVG format to improve scaling, are flat with no gradients, and have in many cases different versions for light and dark mode.

Initially I've set the default icon to be loaded to be the "light" version, as most users will be using that. There's two versions of each icon in each tool directory, even if both icons are actually identical – one with _light appended and one with _dark. This was in the hope that it would make coding the switch easier, as both are always available with a standardized naming convention.

I traced the hand icon in Inkscape, but the original has a Creative Commons license so presumably it is fine to use for an open-source project. It will need attribution though I imagine! I would appreciate it if you checked and sorted the licensing there @ghutchis (assuming you like the icon itself).

Happy for people to add additional code to this PR to make the light/dark switch work, or to have it merge into a different branch.

[github-actions]

Here are the build results
Avogadro2.AppImage
Win64.exe
macOS.dmg
Artifacts will only be retained for 90 days.

[ghutchis]

I haven't had time to write the light / dark switching code, but will add that as a separate pull request.